Changes in intracellular calcium were measured in giant molluscan neurons with dye arsenazo III. Changes in potassium currents were also measured in voltage clamped cells and correlated with intracellular calcium changes under a number of conditions. Our results show that intracellular calcium increases for membrane potentials up to about plus 30 mV and thereafter decreases with a suppression potential of about plus 140 mV in normal external saline. There is a similar increase and decrease in membrane potassium conductance. These increases can be eliminated by external cadmium, nickel, lanthanum, and cobolt. Injection of calcium ions increases intracellular calcium and potassium conductance. This increase in potassium conductance appears to be voltage dependent potassium current in the absence of extracellular calcium. Our results further show tht intracellular barium and cesium but not strontium block the increase in potassium current induced by a change in internal calcium. They show that extracellular tetrealthylammonium, but not intracellular tetraethylammonium blocks the calcium dependent component of the potassium current whereas the voltage dependent component is blocked by tetrealthylammonium on either side of the membrane.